An ICP-MS methodology using a combined high-resolution/multi-collector detector system for the measurement of total zinc and zinc isotope ratios in faecal samples from a human nutrition study

Abstract

Inductively coupled plasma (ICP), using a novel combination of high-resolution and multi-collector mass spectrometric detectors, was used for the determination of zinc totals and zinc isotope ratios in nitric acid digests of freeze dried faecal material. The samples were collected as part of a human study investigating the absorption of iron, copper and zinc from three different diet types. A solution of ⁷⁰Zn was administered to each volunteer and its absorption monitored by measuring the changes to the natural isotopic ratio as the non-absorbed dose appeared in the faeces. ⁷⁰Zn was measured, at a resolution of 2350, on an electron multiplier in the axial channel of the multi-collector array, whilst ⁶⁷Zn, ⁶⁸Zn, ⁶⁹Ga and ⁷¹Ga were simultaneously measured at low resolution (400), using Faraday collectors. Internal and external standardisation procedures were compared for the correction of instrumental drift, but the effectiveness of internal standardisation was limited due to the use of the two different types of detector (electron multiplier and Faraday). Mass bias was corrected for, using the isotope ratio measured in the first faecal sample from each volunteer on the study (t = 0), known to contain only zinc of natural isotopic abundance. The method was validated through the measurement of 190 separate digests of the reference material NIST 1577b, analysed over a seven month period. The mean of all 190 replicates was 125 ± 14 µg g⁻¹, compared to the certified value of 127 ± 16 µg g⁻¹. Zinc isotope ratios measured in the reference material digests showed excellent agreement with published natural isotopic abundance data. Within-sample precision was approximately 0.1 % RSD for ⁶⁷Zn/⁷⁰Zn, ⁶⁸Zn/⁷⁰Zn and ⁶⁷Zn/⁶⁸Zn, and was well inside the requirements of this human study. Good agreement was also found between the isotope ratios measured by TIMS analysis of fully cleaned up samples and by ICP-MS analysis of acid digested solutions of the same set of faecal samples. The preparation and analysis of samples by the ICP-MS method developed allowed a single analyst to digest, dilute and analyse 120 samples in duplicate in a period of 10 working days.